Conventionally, the study of embedding of particular information in image information has grown for the purpose of prevention of unauthorized is duplication and falsification. Such technique is called digital watermarking. For example, a photograph, a picture or the like is electronified, and additional information such as the author's name, permission/prohibition of use and the like is embedded in the image information. A technique which is on its way to becoming standardized in recent years is embedding additional information in original image information in a manner such that the additional information is visually unrecognizable, and delivering the image information via a network such as the Internet.
Further, also being studied is a technique of specifying additional information such as type of printer or its model number from paper on which the image is printed. These techniques are employed for prevention of unauthorized duplication of bank notes, stamps, negotiable instruments and the like, with wide use of high-quality image formation apparatus such as a copier and a printer.
For example, U.S. Pat. No. 5,652,626 discloses a technique of embedding additional information in a high frequency area of visually low-sensible chrominance component and chromaticness component of image.
However, in the above-described conventional art, it is very difficult to embed a large amount of information such as audio information in an image such that the embedded information is not conspicuous when print-outputted.
Accordingly, as means of solution of the above problem, the assignee of the present application has proposed in Japanese Published Unexamined Patent Application No. 2001-148778 (corresponding to U.S. patent application Ser. No. 09/711,956 filed on Nov. 15, 2000) to utilize texture caused by the error diffusion method to artificially generate a combination of quantized values which does not occur in general pseudo halftone processing and embed the generated code in image information. According to this technique, as the shape of texture merely microspically changes, in comparison with the original image, the image quality is not visually degraded. Further, different types of signals can be multiplexed extremely easily by changing a quantization threshold value in the error diffusion method.
Next, a description will be made about an image processing system having an image processing apparatus which embeds additional information in image information and print-outputs an image, and an image processing apparatus which extracts the additional information embedded in a printed image, proposed by the assignee of the present application. FIG. 1A is a block diagram showing the construction of the image processing apparatus proposed by the assignee of the present application which embeds additional information in image information and print-outputs an image.
In the figure, reference numerals 100 and 104 both denote input terminals. Multivalue image information is inputted from the input terminal 100, and additional information to be embedded in the image information is inputted from the input terminal 104. The additional information is different information from the image information inputted from the input terminal 100, e.g., audio information and copyright information regarding the image inputted from the input terminal 100. The additional information is subjected to coding processing for error correction by an error-correction coding unit 103. In the error-correction coding, any type of code e.g. BCH (Bose-Chaudhuri-Hocquenghem) code or Reed-Solomon code may be used. The multiplexed information resulted from the error-correction coding is inputted into an additional information multiplexing unit 101. The additional information multiplexing unit 101 embeds the additional information in the image information in a manner such that the additional information is visually indiscriminable. Further, the additional information multiplexing unit 101 quantizes the input multivalue image information. Numeral 102 denotes a printer which outputs information generated by the additional information multiplexing unit 101 by a printer engine. The printer 102 may be a printer such as an ink-jet printer, a laser printer or the like which realizes tonality representation by using the pseudo halftone processing.
FIG. 1B is a block diagram showing the construction of the image processing apparatus proposed by the assignee of the present application which extracts additional information from a printed image.
Information on a printout is read by using a scanner 105, and additional information embedded in the printout is extracted by an additional information extraction unit 106. The extracted additional information is subjected to error-correction decoding processing by an error-correction decoding unit 107, and outputted from an output terminal 108. Note that as the algorithms regarding multiplexing and decoding are described in Japanese Published Unexamined Patent Application No. 2001-148778 (corresponding to U.S. patent application Ser. No. 09/711,956 filed on Nov. 15, 2000), they are omitted here.
By application of the above method, in comparison with the conventional art, a large amount of information having an arbitrary content can be embedded in an image without degrading the image quality.
However, in the above method, the error-correction coding processing having the same correction capability is performed regardless of the type of printer to print output an image, the resolution upon printing, the print quality arbitrarily set by a user, the characteristic of multiplexed image and the like. Generally, the precision of reproduction of embedded additional information x(j) from a printed image read by the image scanner 105 depends on errors upon reading by the additional information extraction unit 106. The error rate depends on the above-described conditions.
That is, in use of high-performance printer capable of printing an image of quality equivalent to a photograph, as it has a high print precision, the error rate at the additional information extraction unit 106 is low. However, in use of printer which cannot print an image of such high quality, the error rate at the additional information extraction unit 106 is higher. Further, the error rate increases in proportion to the print quality set by the user.
Further, the error rate changes depending on the character of image to be multiplexed. For example, in a case where an image has many high density portions upon printing, blur due to a large amount of ink or toner often causes reading error. On the other hand, if an image has many low density portions, the amount of ink or toner is too small, which often causes reading error due to insufficient representation of periodicity of texture indicating additional information.
In this manner, in the conventional art, the image processing is not performed with distinction between a case where the error rate is high and a case where the error rate is low. That is, the amount of information is increased by reducing the amount of check bits even if it is expected that the error rate is low. Any method of changing the amount of check bits by various conditions has not been proposed.
Further, as uniformly same error-correction coding processing has been performed regardless of characteristic of additional information, the processing is susceptible to improvement. For example, some additional information disables reproduction of the entire information if the additional information includes even 1-bit error information, on the other hand, some additional information has redundancy and does not seriously disturb reproduction of additional information even if the information includes bit error to some degree. As an example of the former information, information of image compression format JPEG can be given, and as an example of the latter information, a BMP file (a file where pixel-unit data are simply rearranged without compression), a WAV file (a file where sound sampled data are simply rearranged) and the like can be given.
That is, regarding a BMP file, a WAVE file and the like, even if more or less bit errors are included, no problem occurs upon representation (reproduction). In other words, in a case where such information is embedded, the intensity for error correction may be low.